Every now and then a new semiconductor material comes along that holds huge promise in a variety of applications. Gallium nitride (GaN) is one such compound, generating interest over the past couple of decades for its unusual capacity to emit green, blue, and ultraviolet light and for its potential use for high-power, high-frequency, and high-temperature devices. Although some devices using gallium nitride laser diodes such as Blu-ray players have already hit the market, much of the potential of gallium nitride has yet to be unlocked.
"Using gallium nitride and its alloys, you can access the entire visible spectrum, ... so from the point of view of optoelectronics, being able to get any wavelength you like makes it a very exciting material system," says Rachel Oliver, a materials scientist at the University of Cambridge in the United Kingdom.
Oliver began working with gallium nitride during her Ph.D. at the University of Oxford in the United Kingdom, where she set out to grow crystals of GaN alloys that are so tiny that they start behaving like atoms -- structures called quantum dots. "We were trying something that we thought maybe was a little mad, something we didn't think was going to work," Oliver says. Yet, when she put her sample in an atomic force microscope to check the outcome of her experiments, "there were my tiny structures," she says.
Seven years later, Oliver, 31, now has a permanent research position at the Cambridge Centre for Gallium Nitride, where she continues her work on this unique and important semiconductor material. Her early success in growing quantum dots in GaN alloys, she says, "has taken me a very long way since."
Photo (top): A scanning electron microscopy image of an atom probe tomography sample, one of the first such samples successfully prepared by Rachel Oliver's lab.
Oliver got exposed to research at a very young age; her father is a biologist. "I can remember being in the lab when I was maybe 6 or 7 and ... being allowed to press buttons and help out with little bits of experiments," Oliver says. But she wasn't inclined to work with cells. Rather, she was curious about things such as why metal feels cold when you touch it, she says: "Those sorts of questions started me thinking about the properties of materials."
Oliver studied engineering and materials science at Oxford in a joint honors degree. Following a series of summer industry placements in material design and metallurgy, she decided to do her final-year research project on sensors for optoelectronic materials. She had become interested in how material properties change at very small scales. "Electronic materials and optoelectronic materials give you a lot of opportunities to access those kinds of effects," she says.
She carried her interest in small structures over to her Ph.D., which she started in 2000 under the supervision of Andrew Briggs in Oxford's Department of Materials. "I was trying to find a way of growing quantum dots in indium nitride or indium gallium nitride with the longer-term goal of finding ways of applying those quantum dots in useful devices," Oliver says.
Oliver first tried using a technique for depositing crystalline-thin films called molecular beam epitaxy, but she quickly grew frustrated because the equipment she had access to proved inadequate for the job. In her third year, her supervisor helped her set up a collaboration with the Cambridge Centre for Gallium Nitride so she could go work in that lab and use another growth technique called metal-organic vapor phase epitaxy (MOVPE). Back then, "there was not much information in the literature really about how to grow quantum dots by MOVPE," she says. "But with the guys [at Cambridge], we did some thinking and came up with ... a really novel way of growing quantum dots."
The work put Oliver's career on a fast track. "I rapidly saw that she was a brilliant researcher and I told her that I would make every effort to obtain funding for her so that she could come to my lab as a postdoc," Colin Humphreys, who leads the Cambridge Centre for Gallium Nitride in the university's Department of Materials Science and Metallurgy, writes in an e-mail to Science Careers.
After Oliver finished her Ph.D., she joined the Cambridge center in October 2003 with postdoctoral fellowships from the U.K. Royal Commission for the Exhibition of 1851 and Peterhouse College in Cambridge. "It gave me a lot more freedom than most postdocs have to explore my own ideas," she says.
In this week's Science: Nitrides Race Beyond the Light
As part of the special section on materials for electronics in this week's issue of Science, Robert F. Service tells how long prized for their optical properties, nitrogen-based semiconductors may take electronic devices into realms where silicon cannot tread.
For her fellowship work, she decided to use the growth technique she had developed during her Ph.D. to try to understand more about the properties of quantum dots and the mechanisms by which they grew. She also set out to develop a single photon source for quantum cryptography, building a nanostructure made of indium gallium nitride (InGaN) crystals in a surrounding gallium nitride matrix "so that the tiny InGaN crystals are like the chocolate chips in a chocolate chip cookie," she says. "If you can get your little crystals, your chocolate chips, tiny enough, ... [their] atom-like behavior allows you to get guaranteed emission of one photon at a time." By the end of her postdoc, together with a collaborator in Oxford, Oliver had designed "the first blue-emitting single photon source," she says.
In another project, Oliver decided to relate her research to one of the main goals of the Cambridge center -- developing thin layers of indium gallium nitride called quantum wells for light-emitting diodes (LEDs). Currently, gallium nitride crystals are mainly grown on a substrate made of another material such as sapphire, a technique that causes "millions and millions more defects in gallium nitride than conventional semiconductors," Oliver says. She focused her interest on a long-standing question: Why are gallium nitride materials in LEDs able to generate so much light when they have all these defects?
One possible explanation came more than a decade ago from transmission electron microscopy pictures that revealed some "blobs" in the indium gallium nitride layers: These were interpreted at the time as regions of high indium content that prevented the charge carriers moving through the LEDs from reaching the defects, which would cause them to generate heat rather than light, Oliver says. But in the early 2000s, the blobs were shown to be "actually caused by the electron beam in the transmission electron microscope," Oliver adds, and "a really big controversy" ensued about what was real and what was not. "So we needed another way of looking at the indium content in these quantum wells."
In collaboration with Alfred Cerezo of Oxford, Oliver developed a new method for looking at gallium nitride materials using atom probe tomography -- a technique that yields a three-dimensional map of a material's composition at the atomic scale. As gallium nitride is not a very conductive semiconductor, this was a tall order, Oliver says. But by the end of her postdoc, she had that technique sorted out, too. She found no blobs of indium in her materials, she says, showing that "the theory that everyone used for several years to understand how these devices work appears not to be true."
Oliver was able to carry on with her work when she took a nonpermanent faculty-level position supported with a 5-year Royal Society University Research Fellowship in October 2006. The fellowship pays for her salary and some research expenses, and Oliver has since put together a small team of three Ph.D. students, one postdoc, and several short-term project students. The fellowship, which has allowed her to prove herself as an independent researcher, has been "an absolutely brilliant ... start to my career," she says.
Then, last year, Oliver landed a permanent faculty position in her department, which she officially started in October. Because she still has 2 years left on her university research fellowship, she immediately took a leave of absence from her permanent position. "Staying on my university research fellowship provides me with some funding and some freedom to do more research and less teaching," she says. She will take on duties as a full faculty member in 2011.
The nature of Oliver's work means she collaborates with researchers around the world and with industry. "I grow samples and characterize them in microscopes, and then I might send them to my collaborators in Oxford or more recently to my collaborators at Harvard University, ... and they will test out the optical properties of my materials and start fabricating devices," she says. "It's always very exciting to hear back from them about the successes they're having with the device development and then to be able to relate that to what I can do to improve the materials."
Her current atom-probe work is done in close collaboration with Sharp Laboratories of Europe, and she speaks regularly with various companies about her single photon sources. She sees an ample niche for academia to complement the research carried out by industry.
"The field is very much driven by devices, and ... technology sometimes gets ahead of our scientific understanding," Oliver says. "If academics are going to add something to this field rather than just kind of follow along in the wake of industry, you need an ability to step back and say, 'Well, what do we really understand about what's going on here, and what information are we missing that might help us to understand this better?' "
And as Oliver works to push her field forward, she's also pushed herself forward on her career path. Oliver's "strengths are her ability to define the key experiments which need to be done and then to perform these and get results," Humphreys says. Outside of being extremely well organized and working hard, "She is ambitious and knows where she wants to go."
Elisabeth Pain is contributing editor for South Europe.